The present invention relates to gaging apparatus for determining deviation of a material property from a nominal value in the range of values and more particularly to an improved structure and arrangement of radiation gage equipment for measuring the weight per unit length of reinforcing bar where the reinforcing bar material is moving past the gage equipment. The weight per unit length measurement is a function of bar dimension, shape and composition and a high energy radiation gage measurement is used to provide a single composite signal accounting for all of these measuring parameters. The principle of radiation absorption in a material upon which this is based, stated simply, is that the intensity of a beam of radiation, as it passes through a material, decreases exponentially as a function of density, thickness and linear absorption factor for the material. Also, weight may be described as a function of density and thickness. Therefore, weight information will be contained in a radiation beam, after passing through a sample varying in shape and density.
Gages in this equipment may be adjusted to read thickness and density besides weight per unit length and other such allied uses.
Such radiation gage equipment has been described in prior art patents such as U.S. Pat. No. 3,832,550 wherein is discussed the use of a single range radiation gage having a source of radiation and a scintillation type of radiation detector operating in an interruptable, self-balancing measuring loop. In the present invention the equipment of the prior art patent has been added to and modified in order to greatly increase the accuracy of measurement over that of the equipment found in the patent and what is presently known and commercially available.
U.S. Pat. No. 3,864,573 also describes a radiation gage utilizing a scintillation detector with a source of nuclear radiation and with a self-balancing measuring loop useful for the purposes of the present invention and also containing some elements of the present invention. But again it will be noted that differences of the structure of the present invention have enabled a more accurate measurement with ancillary benefits.
Most notably the present invention provides a means to gage the weight per foot of reinforcing bar which is greatly simplified over prior art such as described in U.S. Pat. No. 4,057,725, which requires the use of a scanning multiplicity of radiation sources and detectors.
Reinforcing bar is sold on a theoretical weight per foot basis where ASTM standards of weight tolerance permit rolling the product 6.0% lighter than the theoretical weight per foot of a given bar size. Mill performance is limited by present gaging and control techniques wherein weight per foot is based on manual weighing of one foot samples cut from production at specified time intervals. When operated in this manner it is possible that undesirable weight per foot fluctuations could occur on a bar to bar basis and therefore this practice, when a one foot section is manually weighed every fifteen minutes, permits rolling the product an average of only 2.8% light with a two sigma variation of .+-.2.56% for all sizes.
In order to consistently roll reinforcing bar closer to the 6% specification tolerance, a means as in the present invention to continuously measure weight per foot as it exits the finishing stand is required. The prior art does not disclose such a continuously measuring reinforcing bar weight per foot gage structured as in the present invention.